What helps muscles to relax after contraction and tooth enamel retain calcium?

1. Muscles relax after contraction due to the action of calcium pumps in the sarcoplasmic reticulum. These pumps actively transport calcium ions from the cytosol back into the sarcoplasmic reticulum, which lowers the calcium concentration in the cytosol and allows the muscle to relax.

2. Tooth enamel retains calcium due to the presence of hydroxyapatite crystals. Hydroxyapatite is a mineral composed of calcium, phosphorus, and oxygen, and it is the main component of tooth enamel. The calcium ions in hydroxyapatite crystals are tightly bound to the phosphate ions, which prevents them from being dissolved away by acids in the mouth.